1. Field of the Invention
The present invention relates to electrical signal processing, and, in particular, to continuous-time filters implemented with transconductor cells, such as filters having one or more biquadratic filter sections and filters having ladder structures.
2. Description of the Related Art
FIG. 1 shows a block diagram of a prior-art filter 100 used to generate a filtered electrical output signal 106 from an electrical input signal 102. Filter 100 has two biquadratic filter sections 104a and 104b and a tuning circuit 108. Filter section 104a receives and filters input signal 102. Filter section 104b receives and further filters the signal generated by filter section 104a to generate output signal 106. Tuning circuit 108 provides control signals 110 that control the tuning of filter sections 104a–b. 
FIG. 2 shows a block diagram of the architecture of each biquadratic filter section 104 of FIG. 1. As shown in FIG. 2, each biquadratic filter section 104 has four transconductor (gm) cells 202a–d (having transconductances gm1, gm2, −gm3, and −gm4, respectively) and two capacitors C1 and C2. The filter section's input signal Vin is received at the input to gm cell 202a, while the filter section's output signal Vout is presented at the node shared by gm cells 202b and 202d and capacitor C2. The biquadratic filter section of FIG. 2 may be said to have an input node 204 (to which gm cell 202a is connected), an intermediate node 206 (to which capacitor C1 and all four gm cells are connected), and an output node 208 (to which capacitor C2 and gm cells 202b and 202d are connected).
Each biquadratic filter section 104 in FIG. 1 implements a pair of corresponding complex poles located in the complex plane at A±Bj, where the Q of the filter section is given by √{square root over (A2+B2)}/2A.
FIG. 3 shows a schematic circuit diagram of the architecture of the main components of each transconductor cell 202 of FIG. 2. As shown in FIG. 2, the cell's differential input signal Vin+, Vin− is respectively applied to the gates of transistors M1 and M2. The outputs from current sources I1 and I2 are applied to the drains of M1 and M2, respectively, while the sources of M1 and M2 are both connected to the input of current sink I3. The cell's differential output signal Vout+, Vout−appears at the drains of M1 and M2. Common-mode control circuit 302 samples the differential output signal and generates a control signal 304 for current sink 13, e.g., to maintain a desired common-mode voltage level at Vout+ and Vout−.
Referring again to FIG. 1, tuning circuit 108 of filter 100 has an oscillator 112 consisting of two integrators 114a and 114b configured in a loop, where each integrator 114 has a transconductor cell 116 and a capacitor 118. In addition, tuning circuit 108 has a reference oscillator 120, a phase detector 122, and a charge pump/filter 124. Phase detector 122 compares the reference oscillator's output signal 126 to a signal 128 based on the signal oscillating within oscillator 112 to generate UP or DOWN control signals for charge pump/filter 124, depending on whether the phase of oscillation signal 128 lags or leads the phase of reference signal 126. Charge pump/filter 124 accumulates electrical charge corresponding to the UP/DOWN signals from phase detector 122 to generate a tuning control signal 130 used to control the tuning of gm cells 116a–b to drive the oscillation frequency of oscillator 112 to match that of reference oscillator 120. As such, tuning circuit 108 functions as a phase-locked loop (PLL).
The electrical characteristics of transconductor cells 116a–b and capacitors 118a–b are specifically selected to replicate as closely as possible the electrical characteristics of each biquadratic filter section 104 in the main signal path of filter 100. As such, the same control signal (130) used to control the tuning of gm cells 116a–b in oscillator 112 is also applied as control signals 110 to control the tuning of gm cells in each biquadratic filter section 104 (as indicated in both FIGS. 1 and 2).
Although not shown in the figures, an amplitude detector is often employed within tuning circuit 108 to monitor the amplitude of the signal oscillating within oscillator 112 to adjust the output conductance of each gm cell in order to maintain a constant, desired oscillator amplitude. This can provide accurate tuning of the Q of each biquadratic filter section 104 in filter 100.
Notwithstanding the general operability of filter 100, errors will typically result from a number of sources, including differences in parasitic capacitances at the transconductor cell output nodes, random component mismatch, and differences in cell loading. In addition, the reference oscillator in the tuning circuit can create interference with the signal being filtered in the main signal path.